It's not that they were all alive at once but that much life to have lived within that short time is such an impossibility that it shatters any notion of the age of the earth being 6000years old.
For example. To make 1 ft of coal it takes 20feet of dead forest compressed. Nearly the same goes for limestone formation in either solution or compression. Limestone can be compressed even further into marble etc etc etc.
The amount of life to create such strata is mind bogglingly vast.
Okay... so how long did 'creation' take? What was a 'day' for God? Where does the Bible date the earth at only 6000 yrs exactly? (asks the devil's advocate, or in this case, the Bible's... lol)